In some optical applications, there is a need to couple light efficiently between the fundamental modes of two different waveguides, the first and the second waveguide. With respect to waveguiding properties, the two waveguides are characterized by their cross-sections, which can have different sizes, shapes and refractive index differences, or in general, different refractive index distributions. The cross-sections determine the field distributions of the waveguide modes, including the fundamental mode. FIG. 1 represents the core geometries for two typical waveguide types, namely a strip waveguide shown in FIG. 1a and a rib waveguide shown in FIG. 1b. The core is usually surrounded by a lower-index cladding material not shown in FIGS. 1a and 1b, or materials, which can be solid matter, gas or even liquid.
In case of waveguides on planar substrates (i.e. on waveguide chips or wafers), the width W of the waveguide can often be easily modified (e.g. by changing the linewidth in a photolithographic mask), but the waveguide thickness H is much more difficult to manipulate. Often the waveguides are formed by etching evenly deep trenches or by depositing evenly thick material layers on top of the substrate. Continuous changes in layer thicknesses along the propagation direction are often difficult to realize on planar substrates. Therefore, in light coupling between different waveguides the difference in waveguide thicknesses or, in particular, in the vertical intensity distributions of the optical fields, is often more crucial than any differences in the horizontal direction. Therefore, it is essential to find a way to efficiently couple light between thin and thick waveguides, or thin and thick waveguide fields.
In optical systems a single optical signal often propagates through several different waveguides or similar propagating media, which are each optimized for a given optical function. For example, an optical fiber may be used for long-haul transmission, a thick silicon waveguide may be used for efficient coupling with the fiber as well as for low-loss propagation on a silicon chip, and a thin silicon waveguide may be used for realizing miniaturized optical circuits and fast optical modulators on a silicon chip. Also, light emitting diodes, lasers and optical detectors have various different sizes and shapes. These may also have to be coupled to the waveguides, which often influences the design for different waveguide cross-sections. As it is often unpractical to propagate light only along one type of waveguide, low-loss coupling of the fundamental mode between different waveguides is a crucial challenge in realizing optical systems.
It is preferred to realize the first and the second waveguide, as well as the coupling means between them, on a common chip or substrate. Then it is possible to align the waveguides and the coupling means lithographically on a wafer scale, which avoids the typical increase in cost and loss associated with active and passive alignment of separate optical subcomponents and, especially, small waveguides.
Direct butt-coupling, i.e. aligning the two waveguides successively along a common line, is the most straight-forward method for waveguide coupling, but this often results in high coupling losses, especially when the field distributions of the waveguides are clearly different. This method can be successfully used if the field distributions are sufficiently similar, e.g. between an optical fiber (core diameter ˜9 μm) and a size-matched silicon waveguide (width and height ˜9 μm).
Many other methods and coupling structures have been proposed for this important task, each having their own advantages and drawbacks.
One known coupling structure is a horizontal taper with a continuously changing waveguide width. The taper is placed between the first and the second waveguide, so that they are all aligned successively with respect to each other. This method is easy to implement on waveguide chips, but it cannot efficiently compensate for the field-mismatch in the vertical direction. However, it is often used in conjunction with other coupling methods to minimize the horizontal field-mismatch.
Another known coupling structure is a vertical taper with a continuously changing waveguide thickness. This is similar to the horizontal taper, but much more difficult to realize in practise due to the above mentioned limitations in planar processing.
Another known coupling structure is a vertical taper consisting of more than one etch step on each side of the waveguide core, as described e.g. in U.S. Pat. No. 6,108,478, and illustrated in FIG. 2. In such a taper the thickness of the waveguide changes abruptly in the tip of the upper taper, but the vertical field distribution of the fundamental mode changes gradually between the first and the second waveguide. Such a taper can be easily fabricated by etching trenches into a homogeneous silicon-on-insulator (SOI) layer in two or more successive etching steps. However, after such a process the thickness uncertainty of the thinner waveguide is a combination of the uncertanties in the original silicon layer thickness (e.g. +/−500 nm in bonded SOI wafers) and the etch depths (e.g. 5-10%). For many practical applications such an uncertainty is too much. Epitaxial growth of silicon may reduce the thickness uncertainty, but it also increases the complexity and costs in fabrication.
One set of known coupling structures is parallel couplers that couple light between two parallel and different waveguides that are positioned side by side or on top of each other, or in some cases even within each other. Coupling of light with these parallel couplers between two clearly dissimilar waveguide cores can be based on grating assisted coupling, directional coupling or adiabatic coupling. Grating assisted coupling typically involves problems, such as expensive fabrication, high coupling loss, and dependency on wavelength and polarization. Adiabatic coupling means that the optical power does not couple from the fundamental mode to higher-order modes. It requires sufficiently long tapers and couplers (slow transformations in waveguide cross-section along the structure).
Known variations of directional couplers and adiabatic couplers are described e.g. in U.S. Pat. No. 6,282,345 B1, U.S. Pat. No. 6,229,947 B1 and U.S. Pat. No. 6,310,995 B1, and illustrated in FIG. 3. These known couplers are typically fabricated from compound semiconductor materials, such as InP or GaAs, by epitaxial layer growth. In the directional coupler one input signal excites both system modes of the waveguide core pair, and the interference of the modes couples light between the two cores. This coupler is sensitive to its length and also somewhat sensitive to wavelength and polarization. Correspondingly, in the adiabatic coupler one input signal excites only one system mode of the waveguide core pair, and adiabatic transformation of the system mode's field distribution couples light between the two cores. This is schematically illustrated in FIG. 4 for a two dimensional case (solid line=lowest system mode, i.e. the fundamental mode, dotted line=second-lowest system mode). The optical power of the waveguide pair's fundamental mode is mostly confined by the waveguide that has a higher effective index when considered alone, i.e. without the other waveguide. The power ratio between the waveguides reverses around that point where the effective indices cross. It should be noted that in FIG. 4 the widths of the waveguides determine their effective indices, but in a physical three-dimensional structure the effective indices depend on the whole refractive index distribution, including the effects of core width and height, as well as the refractive indices of all the materials. Adiabatic coupling is typically not sensitive to wavelength, but it often requires longer coupling length than interferometric coupling. All of these parallel coupler types, and particularly the known adiabatic coupler structures, represent the technology which is closest to the invention and corresponds to the preambles of claims 1 and 13.
In all the above mentioned vertical tapers and parallel couplers the top surface involves deep trenches or at least high non-planarity in the wafer-scale. This poses severe limitations and difficulties to the further processing of the wafer. For example, patterning of narrow metal strips, such as electrodes or heaters, flip-chip bonding, and hermetic sealing of the chip become more and more difficult as the maximum trench depth increases on top of the wafer. Often such processes need to be done after the waveguide patterning due to restrictions e.g. in process temperatures. It is difficult to pattern anything on top of ribs surrounded by deep trenches and even more difficult to pattern anything to the bottom of deep trenches, surrounded by thick ribs. Nevertheless, there is often a need to pattern metal contacts etc. on top of waveguides, and particularly on top of the thinner waveguides that in the known coupling structures are typically surrounded by much thicker ribs.
In some applications there is a need to seal the top surface of a waveguide chip hermetically, and this can be done by attaching a cap on top of the chip with e.g. direct bonding or glueing. Using any of the known vertical tapers or parallel couplers leaves deep trenches on the surface, which makes the hermetic sealing quite difficult or at least requires some kind of planarization before the cap attachment.
At least in silicon technology, there is a lack of an optical coupler that efficiently couples light between a thin and a thick waveguide, provides good thickness tolerance for the thinner waveguides, enables easy patterning of metal contacts etc. on top of the thinner waveguides, and maintains good planarity of the top surface.